1. Field of the Invention
The present invention relates to a lens system for a video projector, and more particularly to a refractive lens system located in front of the cathode ray tube for projecting onto a screen a video image formed on a face plate of the cathode ray tube.
2. Description of the prior art
In designaing a lens system for a video projector of three-tube type, the correction of chromatic aberration can be generally left out of consideration, since three identical video projector lens systems are located in front of a three cathode ray tubes each forming decomposed three different monochromatic images, respectively, to project them toward a screen for composing one complete color image thereon, and each projector lens system is requested to transmit only such a monochromatic image. Strictly speaking, however, the monochromatic image is practically formed by light of a plurality of different wavelengths within a narrow wavelength band, the width of band depending on the spectral characteristic of fluorescent material of individual cathode ray tube. Especially, the images formed by the cathode ray tubes in charge of green image and blue image, respectively, are rather inadequate to be each regarded as a monochromatic image. Futhermore, the video projector lens system generally utilize plastic lens elements for forming aspheric surfaces thereon. The plastic lens element, however, generally shows a greater degree of variation in reflective index depending on the change in temperature, in comparison with a glass lens element. Therefore, image shifting depending on the change of temperature also causes an unnegligible problem.
Various video projector lens systems have been described in Patents or Patent Applications, such as U.S. Pat. Nos. 3,800,085, 4,348,081 and 4,530,575, Copending U.S. patent application Ser. No. 347,032 filed Feb. 8, 1982 now U.S Pat. No. 4,577,935, and Japanese Laid-Open Patent application Nos. 56-78815, and 59-170812.